This invention concerns silicone copolymer fluids which are useful as hydraulic fluids, and particularly as automotive hydraulic fluids. More precisely, this invention concerns a hydraulic fluid which has a high boiling point and which contains, as a principal agent, a siloxane-oxyalkylene copolymer which is compatible with water and with glycols. One of the conventional hydraulic fluids is a composition containing glycol ether as a base fluid which is used in the brake and clutch systems in automobiles. One of the drawbacks of the hydraulic fluid containing glycol ether as a base fluid is that the glycol ether absorbs moisture from the atmosphere due to its moisture absorbing character, causing reduction in the boiling point of the hydraulic fluid and of the vapor lock temperature. Needless to say, it is a serious drawback for the hydraulic fluid to lose one of its important characteristics.
On the other hand, high-speed operation of automobiles, brought about by improvements in the performance of automobiles and the expansion and maintenance of road networks, has made it imperative to improve automobile hydraulic fluids. Moreover, the environment around the brake fluid has increased in temperature due to the installation of exhaust control systems in automobiles. Thus brake fluid with a high boiling point is in great demand. This property prevents the vapor lock phenomenon which occurs during high-speed driving and during frequent brake use on long downhill roads. The recent establishment of DOT 5 specification that the boiling point of a brake fluid be 260.degree. C. or greater and the wet boiling point be 180.degree. C. or greater (Federal Motor Vehicle Safety Standards and Regulations) was aimed at production of brake fluid with a high boiling point.
For this demand, the hydraulic fluid which contains glycol ether as a base is not satisactory; thus, various studies have been conducted and various hydraulic fluids have been disclosed. Although some of these satisfy the requirement of being high boiling point oils, they are not quite satisfactory when they are evaluated as hydraulic fluids. For example, there is a high boiling point hydraulic fluid using a special polyglycol ether as a starting oil. In this case, there is the drawback that the boiling point decreases easily when it absorbs moisture content from the atmosphere. In addition, a hydraulic fluid with low moisture absorbing character which contains a glycol ester as a base was also developed. However, this is more expensive than the hydraulic fluid which contains glycol ether as a base and is inferior, in terms of viscosity, to the hydraulic fluid which contains glycol ether as a base fluid.
Consequently, many silicone-type hydraulic fluids have recently been proposed. For example, the use of orthosilicate esters was disclosed in Kokai Japanese Patent No. SHo 49[1974]-95925. Since orthosilicates do not exhibit satisfactory stability to hydrolysis, they cannot be used as hydraulic fluids for automobiles. Moreover, the use of amino silicones was proposed in U.S. Pat. No. 3,725,287. However, these materials are relatively expensive and show relatively low wet boiling points, and thus the vapor lock phenomenon tends to occur in high moisture environments. In addition, silicone fluids which are useful as hydraulic fluids and which contain hydrocarbonoxy raicals have been disclosed in U.S. Pat. Nos. 2,834,748; 3,821,114; and 3,833,505. Silicone fluids containing ester-functional radicals have been disclosed in U.S. Pat. Nos. 3,830,744; 3,859,321; and 4,088,591. As with the orthosilicate esters noted above silicone fluids containing hydrocarbonoxy radicals or ester-functional radicals are also susceptible to hydrolysis to various degrees. Dimethyl silicone oil and phenylmethylsilicone oil are known as fluids with high boiling points and low moisture absorbing character. However, since these hydrocarbon silicone oils cause shrinkage of rubber parts of the hydraulic system which are in contact with the hydraulic fluid, the silicone oil may leak out. In addition, there is the drawback that the poor lubricating character of these silicone oils results in abrasion of the metal parts moving in the hydraulic system. For this reason, a silicone oil hydraulic fluid which is prepared by adding a rubber-expanding agent and a lubrication-improving agent to the silicone oils, was proposed in Japanese Patent No. Sho 53[1978]-10102.
The hydrocarbon silicone oil type hydraulic fluids have high boiling points, do not absorb moisture from the atmosphere, exhibit less chemical modification at high temperatures, and minimal viscosity changes with temperature. Thus, they are very promising hydraulic fluids. However, these hydraulic fluids of silicone oil have the following two problems. One of these problems is that when water enters the hydraulic system for some reason, such as when the hydraulic system is washed with water or the hydraulic system is exposed to rainfall, the boiling point is decreased a great deal since the silicone oil is not at all compatible with water, and there is high risk of occurrence of vapor lock phenomena. The other problem is that the hydraulic fluid is not compatible with glycols. That is, if a glycol hydraulic fluid is replaced with a hydrocarbon silicon oil hydraulic fluid in the hydraulic system, the hydraulic system may contain two types of fluids, i.e. glycol hydraulic fluid and silicon oil hydraulic fluid, which are incompatible, thereby requiring that the glycol hydraulic fluid be completely removed. Otherwise, the excellent characteristics of the silicone oil hydraulic fluid are dissipated and the characteristics obtained are the same as those of the glycol hydraulic fluid.